1. Field of the Invention
The present invention relates to a selectively reflective film which includes a liquid crystal composition and is capable of high quality color reproduction and high quality image display and which is used, for example, in displays such as LCDs and constitutes a phase-difference film, a color filter, a circularly polarized light reflector or the like, as well as to a process for producing the selectively reflective film.
2. Description of the Related Art
Color filters used in, for example, color liquid crystal displays or the like are generally composed of red (R), green (G) and blue (B) pixels, and a black matrix arranged therebetween for improving display contrast.
Since an efficient process for readily producing high quality color filters with less waste of raw materials has been desired and since high transmissivity and high color purity are required of color filters, color filters mainly comprising a liquid crystal material (particularly, cholesteric liquid crystal) have widely been studied in recent years.
Since a color filter made mainly of cholesteric liquid crystal is of a type using polarized light, which reflects light having predetermined wavelengths and transmits light of other wavelengths, the color filter can efficiently utilize light and is excellent in transmittance and color purity. Although a spin coating method is usually employed to form a color filter film because uniform thickness can easily be attained, this method involves a large waste of raw materials, and hence is disadvantageous in terms of costs. In consideration of the foregoing, a production process using a photoreactive chiral compound is particularly useful.
If a liquid crystal composition containing a photoreactive chiral compound is used, isomerization of the chiral compound progresses when the compound is patternwise irradiated with light having wavelengths to which the chiral compound is photosensitive, depending on the intensity of the irradiation energy. This causes a change in helical pitch (twist angle of the helix) of the liquid crystal compound, whereby desired selectively reflected color lights can easily be obtained in respective pixels simply by conducting patterning exposure having varied light quantities.
In recent years, the level of performance required of color images is very high, and color filters are especially required to achieve high purity and high resolution.
Other optical films such as phase-difference films and circularly polarized reflectors are required to exhibit a large anisotropy of a refractive index (xcex94n) of film in order to have good optical characteristics. In general, the thicker the film is, a larger value of xcex94n is obtained. However, production of thick films is disadvantageous in view of costs, and thus the trend is toward production of thinner films.
In a case where a film or layer (for example, a color filter film or a phase-difference film) containing a liquid crystal composition is formed by applying coating, an undesirable state occurs in that even if treatment is conducted to align the liquid crystal molecules parallel to a substrate at a substrate side of the film or layer, the inclination angle (pretilt angle) of the liquid crystal molecules continuously changes to that of the thickness direction of the layer, particularly when using a low-molecular weight liquid crystal composition. Such a state is caused by the fact that since one surface of the color filter forming layer faces air, the liquid crystal molecules orient perpendicular to the substrate at the side interfacing with air. Therefore, this layer usually needs be disposed between alignment layers. In a case where the liquid crystal composition is polymerized for use as an optical film, however, it is necessary to peel off at least one of the above-mentioned alignment layers after polymerization in order to achieve a light weight and thinness. Thus, there have been problems in that increased steps are required, such as that of producing and removing the alignment layers, and larger amounts of wastes are produced.
In view of the aforementioned problems, it is therefore an object of the present invention to provide a process for readily producing a selectively reflective film through a simple and easily practiciable aligning treatment. The process avoids a decrease in color hues and in resolution, resulting from blurring of color hues at pixel boundaries, achieves selectively reflected color hues having higher color purity and excellent resolution, and further prevents mask blemish. Another object of the present invention is to provide a selectively reflective film which is obtainable by this process and exhibits high selectively reflected color hues and excellent resolution.
Through intensive research, the inventors have made the following findings regarding optical characteristics (particularly regarding improvements in selectively reflected color hues and in resolution) exhibited by selectively reflective films.
In a process for producing a color filter including a selectively reflective film containing a photoreactive chiral compound using a photoreactive cholesteric liquid crystal composition, it is possible to conduct patterning exposure once through a mask having a different light transmittance. However, when using the liquid crystal composition (layer) in a liquid crystal state, during a process after exposure until the layer is hardened via irradiation, the chiral agent, isomerized by different illumination intensities for respective pixels, is likely to diffuse between adjacent areas (colored pixels), whereby color hues are blurred at pixel boundaries causing a decrease in color purity and in resolution. Moreover, at the time of patterning exposure, it is necessary to arrange the mask as closely as possible to the composition (layer). Accordingly mask blemish occurs while the liquid crystal composition is in a liquid crystal state, and hence sufficient resolution cannot be obtained at portions where the mask is not adhered closely to the composition.
In view of the foregoing, the objects stated above are attained by imagewise irradiating a liquid crystal composition (layer) with light while the liquid crystal composition is in a substantially non-liquid crystal phase state.
Accordingly, a first aspect of the present invention provides a process for producing a selectively reflective film comprising the steps of:
forming, on a substrate, a liquid crystal composition layer exhibiting a smectic phase state, a microcrystalline state or a glass state, the liquid crystal composition containing a photoreactive chiral compound, a polymerization initiator, and a liquid crystal compound having at least one polymerizable group;
establishing a distribution of exposure in the liquid crystal composition layer by irradiating the liquid crystal composition layer exhibiting the smectic phase state, the microcrystalline state or the glass state with a first light;
forming a selectively reflective area in accordance with the distribution of exposure by raising the temperature of the liquid crystal composition layer, in which the distribution has been established, such that the liquid crystal composition is transformed to a liquid crystal state; and
hardening the liquid crystal composition by irradiating the liquid crystal composition layer, in which the selectively reflective area has been formed, with a second light to cause polymerization or cross-linking.
A second aspect of the present invention provides a selectively reflective film obtained by a production process comprising the steps of:
forming, on a substrate, a liquid crystal composition layer exhibiting a smectic phase state, a microcrystalline state or a glass state, the liquid crystal composition containing a photoreactive chiral compound, a polymerization initiator, and a liquid crystal compound having at least one polymerizable group;
establishing a distribution of exposure in the liquid crystal composition layer by irradiating the liquid crystal composition layer exhibiting the smectic phase state, the microcrystalline state or the glass state with a first light;
forming a selectively reflective area in accordance with the distribution of exposure by raising the temperature of the liquid crystal composition layer, in which the distribution has been established, such that the liquid crystal composition is transformed to a liquid crystal state; and
hardening the liquid crystal composition by irradiating the liquid crystal composition layer, in which the selectively reflective area has been formed, with a second light to cause polymerization or cross-linking.
In the selectively reflective film according to the second aspect of the present invention, i.e., the selectively reflective film obtained by a production process comprising the steps of: forming, on a substrate, a liquid crystal composition layer exhibiting a smectic phase state, a microcrystalline state or a glass state, the liquid crystal composition containing a photoreactive chiral compound, a polymerization initiator, and a liquid crystal compound having at least one polymerizable group; establishing a distribution of exposure in the liquid crystal composition layer by irradiating the liquid crystal composition layer exhibiting the smectic phase state, the microcrystalline state or the glass state with a first light; forming a selectively reflective area in accordance with the distribution of exposure by raising the temperature of the liquid crystal composition layer, in which the distribution has been established, such that the liquid crystal composition is transformed to a liquid crystal state; and hardening the liquid crystal composition by irradiating the liquid crystal composition layer, in which the selectively reflective area has been formed, with a second light to cause polymerization or cross-linking, the liquid crystal compound exhibits helical structure and the helical structure is fixed in a state such that a helical axis is oriented substantially parallel to the direction of the normal line of the surface of the selectively reflective film when the liquid crystal compound undergoes polymerization or cross-linking. This selectively reflective film is an optical film which displays selectively reflective wavelengths (colors) ranging from ultraviolet light to infrared light.
As used herein, the wording xe2x80x9cthe liquid crystal layer exhibits a smectic phase state, a microcrystalline state or a glass statexe2x80x9d (hereinafter occasionally referred to as a xe2x80x9cnon-liquid crystal statexe2x80x9d or xe2x80x9cnon-liquid crystal phasexe2x80x9d) means that the liquid crystal composition is in the state of a substantially non-liquid crystal phase, such as a crystal state or an amorphous state, and that the components present in the composition, such as the liquid crystal compound and the chiral compound, exhibit a polycrystal state. In particular, the smectic phase state refers to a state having a structure in which the molecules are aligned in a layered form and exhibiting higher viscosity than a nematic phase, due to a higher order than that of a nematic liquid crystal and a very low level of fluidity.
Accordingly, the non-liquid crystal state does not substantially exhibit any structural change (change in helical pitch) effected by the photoreactive chiral compound through isomerization by light. That is, the non-liquid crystal state does not exhibit cholesteric properties (selective reflectivity).